Effects of the analgesic agent tramadol in normal and arthritic rats: comparison with the effects of different opioids, including tolerance and cross-tolerance to morphine

Development of opioid analgesic tolerance in rat to extended-release buprenorphine formulated for laboratory subjects

Buprenorphine in an extended-release formulation intended for use in laboratory subjects is frequently administered to rats to provide extended analgesia without repeated handling. While levels of buprenorphine may persist in serum once extended-release buprenorphine has been introduced, exposure to opioids can cause opioid tolerance or opioid-induced hypersensitivity. This work examined the analgesic duration and efficacy of a single administration of extended-release buprenorphine intended for use in laboratory subjects in models of inflammatory pain and post-operative pain and the development of opioid tolerance in rat. After subcutaneous administration of 1 mg/kg extended-release buprenorphine, analgesic efficacy did not persist for the expected 72 hours. No changes were observed in mechanical thresholds in the hindpaws that were contralateral to the injury, suggesting a lack of centrally mediated opioid-induced hypersensitivity. To determine whether opioid tolerance arose acutely after one exposure to extended-release buprenorphine, we conducted the warm water tail flick assay; on Day 1 we administered either saline or extended-release buprenorphine (1 mg/kg) and on Day 3 we quantified the standard buprenorphine dose-response curve (0.1-3 mg/kg). Rats previously given extended-release buprenorphine displayed decreased analgesic responses after administration of standard buprenorphine as compared to the robust efficacy of standard buprenorphine in control subjects. Males appeared to show evidence of acute opioid tolerance, while females previously exposed to opioid did not demonstrate a decreased response at the doses examined. Taken together, these results suggest that opioid tolerance arises quickly in male rats after exposure to the extended-release formulation of buprenorphine. This tolerance may account for the brief period of antinociception observed.

Benzimidazole Derivative Ameliorates Opioid-Mediated Tolerance during Anticancer- Induced Neuropathic Pain in Mice

BACKGROUND

Cancer is known to be the second significant cause of death worldwide. Chemotherapeutic agents such as platinum-based compounds are frequently used single-handedly or accompanied by additional chemotherapies to treat cancer patients. Chemotherapy-induced peripheral painful neuropathy is seen in around 40% of patients who are treated with platinum-based compounds, including cisplatin. This not only decreases the quality of life of patients but also patients' compliance with cisplatin.

OBJECTIVES

Nalbuphine, an opioid, is frequently used to treat acute and chronic pain, coupled with cisplatin in cancer patients. However, long term use of nalbuphine induces tolerance to its analgesic effects. We employed the same strategy to induce tolerance in mice.

METHODS

Here, we investigated analgesic effects of 2-[(pyrrolidin-1-yl) methyl]-1H-benzimidazole (BNZ), a benzimidazole derivative, on nalbuphine-induced tolerance during cisplatin-induced neuropathic pain using hot plate test, tail-flick tests and von Frey filament in mouse models. Furthermore, we investigated the effects of BNZ on the expression of Tumor Necrosis Factor-alpha (TNF-α) in the spinal cord.

RESULTS

The results showed that BNZ reduced tolerance to analgesic effects of nalbuphine and TNF-α expression in mice.

CONCLUSION

BNZ could be a potential drug candidate for the management of nalbuphine-induced tolerance in cisplatin-induced neuropathic pain.

Analysis of Animal Well-Being When Supplementing Drinking Water with Tramadol or Metamizole during Chronic Pancreatitis

Simple Summary

Pain management during in vivo experiments can considerably improve the wellbeing of animals. However, often it is not clear, which drugs are best for the animals and how to apply these drugs without causing stress. In this study, we evaluated mice when metamizole or tramadol was provided via drinking water. Neither of these two drugs reduced the amount of consumed water or body weight in healthy mice or influenced their natural behavior, such as nest building or burrowing activity. Both analgesics were then given to mice suffering from chronic pancreatitis. Mice drinking tramadol supplemented water, at some time-points, experienced less loss in body weight and consumed more water than mice drinking metamizole. However, no major differences in other methods measuring wellbeing of mice was observed. In conclusion, both analgesics can be used during chronic pancreatitis, but tramadol seems to be moderately advantageous when compared to metamizole.

Abstract

Pain management during in vivo experiments is an animal welfare concern and is in many countries also legally required. In this study, we evaluated C57Bl/6J mice when 3 g/L metamizole or 1 g/L tramadol was provided via drinking water, before and during cerulein-induced chronic pancreatitis. Supplementation of drinking water with metamizole or tramadol did not significantly reduce the amount of consumed water. In order to evaluate the wellbeing of mice, a distress score, burrowing activity, nesting behavior, and body weight was assessed. Before induction of pancreatitis, neither tramadol nor metamizole influenced these readout parameters. Chronic pancreatitis caused a significantly increased distress score, decreased burrowing activity and a reduction in body weight. Mice drinking tramadol-supplemented water experienced less loss in body weight and consumed more water than mice drinking metamizole, at a few time-points during chronic pancreatitis. Pancreatic atrophy, a characteristic feature of chronic pancreatitis was not differentially influenced by either analgesic. In conclusion, both analgesics can be used during 33 days of chronic pancreatitis, but tramadol seems to be moderately advantageous when compared to metamizole.

Affective state determination in a mouse model of colitis-associated colorectal cancer

Behavioural indicators of affective state, including burrowing, clinical scores and the Mouse Grimace Score have not yet been validated in mouse models of chronic gastrointestinal disease. Additionally, a comparison of these methods has not been characterised. This study aimed to determine which behavioural assessment was the optimal indicator of disease, evidenced by correlation with clinically-assessed measures, in an azoxymethane (AOM)/dextran sulphate sodium (DSS) mouse model of colitis-associated colorectal cancer. C57BL/6 mice were allocated to four groups (n = 10/group); 1) saline control, 2) saline+buprenorphine, 3) AOM+DSS+water, 4) AOM+DSS+buprenorphine. Mice were gavaged thrice weekly with water or buprenorphine (0.5mg/kg; 80μL) for 9 weeks. Disease activity index (DAI) was measured daily; burrowing and grimace analyses occurred on days -1, 5, 19, 26, 40, 47 and 61. Colonoscopies were performed on days 20, 41 and 62. All animals were euthanized on day 63. Burrowing activity and retrospective grimace analyses were unaffected (P>0.05), whilst DAI was significantly increased (P<0.05) in mice with colitis-associated colorectal cancer compared to normal controls. In addition, DAI was positively correlated with colonoscopically-assessed severity and tumour number (P<0.05). We conclude that traditional measures of DAI or clinical scoring provide the most reliable assessment of wellbeing in mice with colitis-associated colorectal cancer.

Intrathecally injected tramadol reduces articular incapacitation and edema in a rat model of lipopolysaccharide (LPS)-induced reactive arthritis

AIMS

Intrathecal injection of morphine presents analgesic and antiedematogenic effects in rats. However, it is unknown whether tramadol, which possess a mixed mechanism of action, can also produce analgesic and antiedematogenic effects similarly.

MAIN METHODS

Male Wistar rats received carrageenan and LPS in the right knee joint. Tramadol (10 μg) was injected intrathecally 20 min before articular LPS injection. Incapacitation and articular edema were measured 5 h after LPS stimulation. Synovial fluid was collected for leukocyte counting and western blot analysis. Whole joint and lumbar spinal cord were also collected for histology and immunohistochemistry, respectively. Intrathecal pretreatments groups were with the NKCC1 blocker bumetanide, TRPV₁ agonist resiniferatoxin, μ-opioid receptor antagonist CTOP and serotonergic neurotoxin 5,7-DHT, all previously to tramadol.

KEY FINDINGS

Tramadol treatment caused the reduction of incapacitation and edema. It also reduced c-Fos protein expression in the spinal cord dorsal horn and slightly reduced TNF-α levels in synovial fluid, but neither reduced cell migration nor tissue damage. Bumetanide and resiniferatoxin prevented the analgesic and antiedematogenic effects of tramadol. CTOP prevented the analgesic and the antiedematogenic effects, but 5,7-DHT prevented only tramadol-induced analgesia.

SIGNIFICANCE

Spinal NKCC1 cotransporter and peptidergic peripheral afferents seem to be important for the analgesic and antiedematogenic effects of tramadol, as well as μ-opioid receptor. However, the monoamine uptake inhibition effect of tramadol seems to be important only to the analgesic effect.

Acute pain management in the opioid-tolerant patient

SUMMARY Opioid use is increasing worldwide leading to an increasing number of opioid-tolerant patients requiring acute pain management after surgery, trauma and acute diseases. Provision of analgesia in opioid-tolerant patients is complex due to the pharmacological effects of long-term opioid exposure, but also due to pre-existing pain states, comorbidities and psychosocial issues. Acute pain management in these patients is governed by the principles of provision of good analgesia, avoidance of withdrawal and organized discharge. Pain relief needs to be achieved by the use of multimodal analgesia, including regional anesthetic techniques and, if needed, opioids in increased doses. Withdrawal is best prevented by ongoing opioid substitution at previously established doses. Discharge planning requires multidisciplinary input and good communication with all healthcare providers involved.

[Systemic clonidine versus opioids in postoperative analgesia-A randomized double-blind study.]

INTRODUCTION

alpha(2)-Adrenozeptoragonisten agonists have shown antinociceptive and analgesic effects, which are not antagonized by naloxone. Therefore, the mechanism of action should be independent of opioid receptors. Most studies on this topic have been performed using clonidine. Experimentally the analgesic effect of clonidine can be suppressed by the inhibition of central adrenergic receptors. Furthermore, clonidine has analgesic effects at the spinal level. During recent years numerous studies have shown the analgesic effect of spinally or epidurally administered clonidine in humans. However, only very few studies have investigated the analgesic effect of parenterally administered clonidine in humans.

METHODS

After the approval of the local ethical committee had been obtained, 60 patients (ASA I-III, age 18-65 years) scheduled for elective orthopaedic procedures were included in this double-blind randomized study. All patients gave their written consent on the day before the operation. Premedication was standardized and involved benzodiazepines. Isoflurane was used as the sole anaesthetic. Postoperatively the pain level of the patients was controlled by a visual analogue scale (VAS 0-10). When the VAS reached at least 5 and the patients requested an analgesic, they were randomly assigned to either the morphine, tramadol or clonidine group. Twenty patients received 5 mg morphine i.v., 20 patients received 50 mg tramadol and 20 patients received 150 clonidine i.v. If the analgesic effect was insufficient, the above-mentioned dosage was repeated after 30 min. The therapy was classified as a failure if no sufficent analgesia could be achieved within 60 min. These patients received 7.5 mg piritramide i.v. VAS and sedation were measured at 10-min intervals during the 1 st h and at 15-min intervals during the following 2h. Heart rate, blood pressure and oxygen saturation were measured at 5-min intervals during the whole study period. Statistical analysis of the data was performed by ANOVA, Wilcoxon test, Student'st-test and chi-square test using a level of significance ofP<0.05.

RESULTS

All groups were comparable as regards their basic clinical parameters. Morphine, tramadol and clonidine significantly reduced the VAS within 20 min. During the whole study period the analgesic effect of clonidine was comparable with that of morphine and tramadol. No significant differences were observed in the number of repetitions after 30 min or in the failure rate. After 2 h sedation was significantly higher in the morphine group. No clinically relevant cardiovascular or respiratory side-effects were observed in any of the patients.

DISCUSSION

In our study the analgesic effect of 150 mug clonidine i.v. was equivalent to that of 5 mg morphine i.v. and 50 mg tramadol. Our results in humans confirm the dosage relationship of 1ratio30 found by Eisenach in sheep. Further studies on the use of parenteral clonidine for postoperative analgesia seem to be warranted.

Tramadol and its enantiomers differentially suppress c-fos-like immunoreactivity in rat brain and spinal cord following acute noxious stimulus

Tramadol hydrochloride, (1RS,2RS)-2-[(dimethylamino)methyl]-1-(3-methoxyphenyl)-cyclohexanol hydrochloride, is an orally-active, centrally-acting analgesic with a putative dual mechanism of action, including an opioid and non-opioid component. The analgesic properties of tramadol and the possible co-existence of dual mechanisms has been postulated to be due to complementary and interactive pharmacologies of its enantiomers. We examined the ability of tramadol, its enantiomers, and morphine as reference to suppress c-fos-like immunoreactivity (c-fos-ir) in rat spinal cord and brain regions following a noxious stimulus (i.p. administration of 3.5% acetic acid). c-fos-ir was measured by immunocytochemistry and the stained cells in each region were counted 2 h after the acetic-acid injection (2:25 h after tramadol or morphine). Equi-analgesic doses of s.c. morphine (10 mg/kg) or tramadol (30 mg/kg) significantly suppressed c-fos-ir in all areas examined, except dorsal central gray of the spinal cord. The enantiomers of tramadol had distinctive patterns of suppression, neither one suppressed c-fos-ir in all of the regions, and hence neither one alone accounted for the suppression produced by the racemate. These findings support differential and complementary effects of tramadol enantiomers in sub-populations of spinal and supraspinal nociceptive neurons, consistent with the proposed antinociceptive interaction between the enantiomers.

Altered nociceptive C fibre input to primary somatosensory cortex in an animal model of hyperalgesia

Evaluating potentially analgesic effects of drugs and various treatments is critically dependent on valid animal models of pain. Since primary somatosensory (SI) cortex is likely to play an important role in processing sensory aspects of pain, we here assess whether monitoring SI cortex nociceptive C fibre evoked potentials can provide useful information about central changes related to hyperalgesia in rats. Recordings of tactile and CO(2)-laser C fibre evoked potentials (LCEPs) in forelimb and hind limb SI cortex were made 20-24h after UV-B irradiation of the heel at a dose that produced behavioural signs of hyperalgesia. LCEPs from irradiated skin increased significantly in duration but showed no significant change in magnitude, measured as area under curve (AUC). By contrast, LCEPs in hind limb SI cortex from skin sites nearby the irradiated skin showed no increase in duration or onset latency but increased significantly in magnitude after UV-B irradiation. The LCEPs in forelimb or hind limb SI cortex elicited from forelimb skin did not change in magnitude, but were significantly delayed in hind limb SI cortex. Tramadol, a centrally acting analgesic known to reduce hyperalgesia, induced changes that counteracted the changes produced by UV-B irradiation on transmission to SI cortex from the hind paw, but had no significant effect on time course of LCEPs from forelimb skin. Tactile evoked potentials were not affected by UV-B irradiation or tramadol. We conclude that altered sensory processing related to hyperalgesia is reflected in altered LCEPs in SI cortex.

Effects of prolonged treatment with the opiate tramadol on prodynorphin gene expression in rat CNS

A low abuse liability is reported for tramadol, an analgesic drug centrally acting through either opioid or nonopioid mechanisms. In this paper, we evaluated the effects of the repeated administration (7 d) of different doses of tramadol (10, 20, and 80 mg/kg, intraperitoneally) on the opioid precursor prodynorphin biosynthesis, in comparison with morphine (10 mg/kg, intraperitoneally), in the rat central nervous system (CNS). Northern analysis showed that morphine and tramadol produced different effects. While morphine caused a downregulation of prodynorphin mRNA levels in all investigated areas (hypothalamus, hippocampus, and striatum), tramadol did not cause any significant change in the striatum, and did not decrease prodynorphin biosynthesis in the hypothalamus and in the hippocampus, at nontoxic doses (10 and 20 mg/kg). The highest dose of tramadol (80 mg/kg) decreased prodynorphin mRNA levels in the hypothalamus and the hippocampus but not in the striatum. These data give some information on tramadol effects at molecular level in the CNS. They indicate that the alterations of prodynorphin gene expression caused by tramadol and morphine show a different pattern that may be related to the different abuse potential of the two analgesic drugs.

Synergic Antinociceptive Interaction between Tramadol and Gabapentin after Local, Spinal and Systemic Administration

The possible interaction between tramadol and gabapentin on formalin-induced nociception in the rat was assessed. Tramadol, gabapentin or a fixed-dose ratio combination of gabapentin and tramadol were administered peripherally, spinally and orally to rats, and the antinociceptive effect was determined in the 1% formalin test. Isobolographic analyses were used to define the nature of the interactions between drugs. Tramadol, gabapentin and tramadol-gabapentin combinations produced a dose-dependent antinociceptive effect when administered locally, spinally or orally. ED30 values were estimated for the individual drugs and isobolograms were constructed. Theoretical ED30 values for the combination estimated from the isobolograms were 126.8 +/- 11.1 microg/paw, 23.1 +/- 2.6 microg/rat, and 2.23 +/- 0.32 mg/kg for the local, intrathecal and oral routes, respectively. These values were significantly higher than the actually observed ED30 values which were 13.3 +/- 2.1 microg/paw, 8.1 +/- 0.6 microg/rat and 0.71 +/- 0.10 mg/kg, indicating a synergistic interaction. Although efficacy was not improved, local peripheral administration resulted in the highest increase in potency, being about tenfold. Spinal and systemic administration increased potency threefold. Data indicate that low doses of the tramadol-gabapentin combination can interact synergistically to reverse formalin-induced nociception and may represent a therapeutic advantage for clinical treatment of inflammatory pain.

Interactions of acute morphine with chronic imipramine and fluvoxamine treatment on the antinociceptive effect in arthritic rats

This study was undertaken to investigate the effects induced by chronic systemic administration of two different antidepressants: imipramine (IMI), a dual serotonin-noradrenaline reuptake inhibitor, and fluvoxamine (FVX), a selective serotonin reuptake inhibitor, on the antinociceptive effect of morphine (MOR) in a paw pressure test in adjuvant-induced arthritic rats. For 30 days rats were administered with IMI, FVX or saline (SAL). On days 15 and 30, animals were tested in the paw pressure test 20 min after MOR or SAL administration. MOR induced a significant antinociceptive effect in IMI, FVX and SAL treated rats. But, at 30 days, this increase in pain threshold was significatively higher in IMI than SAL rats. This increase was not seen in FVX rats. These results suggest that a combination of opioid and mixed monoaminergic activities is effective in enhancing the antinociceptive effect of MOR in arthritic rats while only opioid and serotonergic activities have no enhancer effect.

Differential tramadol and O-desmethyl metabolite levels in brain vs. plasma of mice and rats administered tramadol hydrochloride orally

OBJECTIVE

To investigate a possible differential brain uptake of tramadol vs. its major metabolite (O-desmethyl tramadol; M1) in mice and rats.

METHODS

An extraction and measurement technique (gas chromatograph equipped with a nitrogen phosphorus detector) was used to measure plasma and brain levels of tramadol and M1 at intervals 10-300 min after oral dosing of tramadol hydrochloride to mice and rats.

RESULTS

For all doses of tramadol administered (5, 10, 20, or 40 mg/kg), tramadol and M1 plasma levels were greatest 10 min after dosing: in mice, peak tramadol plasma levels were 47.75-736.72 ng/mL and peak M1 levels were 75.30-1084.92 ng/mL; in rats, peak tramadol plasma levels were 185.03-455.81 ng/mL and peak M1 levels were 106.74-455.70 ng/mL. Tramadol brain levels were also greatest 10 min after dosing. In mice, peak tramadol brain levels were 226.42-1847.46 ng/g. Peak M1 levels (72.17-572.97 ng/g) occurred 20-60 min after dosing. In rats, peak tramadol brain levels were 258.50-1777.37 ng/g and peak M1 levels were 80.35-289.60 ng/g. In mice, the ratio of tramadol/M1 in plasma was 0.5-1.0 throughout the measurements, whereas the ratio in brain was about 10 at 10 min and about 2 from 20 to 50 min. In rats, the ratio of tramadol/M1 in plasma was 0.5-1.5, whereas the ratio in brain was about 15 at 10 min and about 4-7 thereafter.

CONCLUSION

In mice and rats, there appears to be preferential brain vs. plasma distribution of tramadol over M1.

Effects of tramadol on T lymphocyte proliferation and natural killer cell activity in rats with sciatic constriction injury

We investigated the effects of acute and chronic tramadol treatment on T lymphocyte function and natural killer (NK) cell activity in rats receiving chronic constriction injury (CCI) of the sciatic nerve. T lymphocyte function was evaluated based on concanavalin-A (ConA)- and phytohemagglutinin (PHA)-induced splenocyte proliferation. NK cell activity was measured by lactic acid dehydrogenase release assay. The effects of tramadol on thermal hyperalgesia were also assessed by measuring paw withdrawal latency (PWL) in the rats. PWL was dose-dependently reversed by tramadol after acute treatment (single subcutaneous injection) with 10, 20, and 30 mg/kg, respectively. There was no significant change among acute treatment groups in NK cell activity, whereas splenocyte proliferation induced by ConA and PHA was significantly suppressed starting from a dose of 20 mg/kg. The reversal of the thermal hyperalgesia persisted throughout a period of chronic tramadol treatment of 40 and 80 mg/kg per day, respectively, with continuous subcutaneous infusion for 7 days at a uniform rate via osmotic minipumps. No modulation of NK cell activity was found in either dose group. However, the activity of splenocyte proliferation was decreased in the 80 mg/kg per day group when compared with the saline and 40 mg/kg per day groups. These data suggest that tramadol treatment has an immunological profile different from pure mu-opioid agonists like morphine, which is known to suppress both NK cell activity and T lymphocyte proliferation at a subanalgesic dose in CCI rats. Considering analgesic and immunosuppressive effects, tramadol treatment may be a better choice than morphine for treatment of chronic neuropathic pain, particularly in patients with compromised immunity.

Pain Processing: Paradoxes and Predictions

During the last 25 years, there have been substantial advances in our understanding of the physiology and pathophysiology of pain. The development of animal models that more closely mimic clinical pain in humans has helped elucidate the putative mechanisms by which chronic pain develops and is maintained. However, our increased understanding of the neurobiology of pain has not translated into breakthrough treatments for pain management. As such, chronic pain is still primarily managed by drugs whose primary indication does not include pain (eg, antidepressants, anticonvulsants, antiarrhythmics, local anesthetics). These adjuvant analgesics have come into favor despite the fact that the mechanisms through which these drugs provide pain relief remain either largely unknown or are not selective for a single target. Moreover, the efficacy of adjuvant analgesics in animal models of pain is often validated only after case studies or clinical trials have been reported. This retrospective validation of "novel" analgesics in animal models of pain raises a question of the predictive validity of these models. This article reviews the use of several adjuvant and standard analgesics currently used to treat difficult-to-manage pain. What can these drugs teach us about the development of novel pain medicines? Within this context, the use of animal models of pain to predict analgesic efficacy in clinical pain conditions is considered.

The antinociceptive effect of tramadol on a model of neuropathic pain in rats

The antinociceptive activity of tramadol was investigated on the vocalization threshold to paw pressure in a rat model of unilateral mononeuropathy produced by loose ligatures around the common sciatic nerve. Despite the analgesic activity of tramadol was clearly established in motor and sensory responses of the nociceptive system in rats, the effect of this atypical opioid on experimental neuropathic pain models is not investigated. The intraperitoneally injected tramadol (2.5, 5 and 10 mg/kg) produced a potent and dose-dependent antinociceptive effect on both lesioned and non-lesioned hind paws. However, the analgesic effect on the lesioned paw was significantly more potent than the non-lesioned paw. This effect was partially antagonized by intraperitoneally administered naloxone (0.1 mg/kg) suggesting an additional non-opioid mechanism. Our results suggest that tramadol may be useful for the alleviation of some symptoms in peripheral neuropathic conditions

Epidural tramadol for postoperative pain after Cesarean section

PURPOSE

To compare the post-operative analgesic effect of 100 mg vs 200 mg epidural tramadol and saline in patients undergoing elective Cesarean section.

METHODS

Sixty healthy women undergoing Cesarean delivery with epidural anesthesia were randomly allocated into three groups (n = 20 in each). Patients received, at skin closure via the epidural catheter, 100 mg tramadol (Group I), 200 mg tramadol (Group II) or 10 ml saline (Control group). Pain scores and side effects were evaluated at 1, 2, 4, 8, 12 and 24 hr after surgery. Mean times to the first analgesic administration, as well as the cumulative doses of analgesic requirements over 24 hr postoperatively were compared.

RESULTS

The mean time to first analgesic administration was longer in patients who received 100 mg tramadol (4.5 +/- 3.1 hr) and the 200 mg tramadol (6.6 +/- 3.4 hr) than in those who received placebo (2.8 +/- 2 hr). The mean cumulative doses of meperidine over 24 hr were less in the 100 mg tramadol group (0.3 +/- 0.3 mg x kg(-1)) and the 200 mg tramadol group (0.3 +/- 0.3 mg x kg(-1)) than in the control group (0.7 +/- 0.4 mg x kg(-1)). Also, the mean doses of diclofenac over 24 hr were less in the 100 mg tramadol group (156 +/- 59 mg) and the 200 mg tramadol group (142 +/- 62 mg) than in the control group (214 +/- 70 mg). However, no difference was obtained between patients receiving 100 mg and 200 mg tramadol concerning all parameters studied.

CONCLUSION

Epidural tramadol 100 mg can provide adequate postoperative analgesia without respiratory depression in patients after Cesarean delivery.

Effect of acute and chronic tramadol on [3H]-norepinephrine-uptake in rat cortical synaptosomes

1 Tramadol hydrochloride is a centrally acting opioid analgesic whose efficacy and potency is only five to ten times lower than that of morphine. Opioid, as well as non-opioid mechanisms, may participate in the analgesic activity of tramadol. 2 [3H]-NE uptake in isolated rat cortical synaptosomes was studied in the presence of tramadol, desipramine, methadone, and morphine. Desipramine and tramadol inhibited synaptosomal [3H]-NE uptake with apparent Kis of 7.3 +/- 0.66 and 1.4 +/- 0.0045 microM, respectively. Methadone was active at a 10-fold higher concentration (Ki: 87 +/- 5.6 microM). In contrast, morphine essentially failed to inhibit [3H]-5-HT uptake (Ki: 0.75 +/- 0.40 M). 3 Methadone, morphine, and tramadol were active in the hot plate test with ED50s of 6.2, 9.3, and 40 mg kg-1, respectively. 4 [3H]-NE uptake was examined in synaptosomes prepared from rats 30 min after receiving a single dose of morphine, methadone or tramadol. Only tramadol (31 mg kg-1, i.p.) decreased uptake of the transmitter, with an ED50 equal to that in the hot plate test. 5 Animals were chronically treated for 15 days with increasing doses of tramadol (20 to 125 mg kg-1, i.p.). Twenty-four hours after the last drug injection, a challenge dose of tramadol (40 mg kg-1, i.p.) was administered. Chronic tramadol was still able to reduce [3H]-NE uptake by 35%. 6 These results further support the hypothesis that [3H]-NE uptake inhibition may contribute to the antinociceptive effects of tramadol. The lack of tolerance in [3H]-NE uptake, together with the absence of behavioural alteration after chronic tramadol treatment proposes that tramadol holds potential over classical opioids in the treatment of pain disorders.

Antinociception, tolerance, and physical dependence comparison between morphine and tramadol

The mechanism of action of tramadol includes the activation of opioid receptors, and the potential ability of the drug to induce tolerance and physical dependence has been evaluated in different animal species and humans. This work was designed to study the involvement of opioid receptors in the antinociceptive activity and the potential ability to develop tolerance, crosstolerance, and/or physical dependence of tramadol. The writhes induced by acetic acid administration was used as algesiometric test. After chronic administration of tramadol, tolerance was evaluated by measuring the antinociceptive activity, and physical dependence was measured by naloxone administration. Morphine was used as drug of comparison. The i.p. administration of tramadol produced a dose-dependent antinociception with an ED50 value of 7.82 +/- 1.16 mg/kg, which was unchanged after chronic administration of either tramadol (39.1 or 100 mg/kg) or morphine (1.05 or 100 mg/kg). By contrast, the ED50 for morphine (0.21 +/- 0.08 mg/kg) was significantly reduced only by chronic pretreatment with both doses of morphine (tolerance). Physical dependence was developed only in mice pretreated with morphine, as evidenced by the presence of jumps, wet-dog shakes, tachypnea, piloerection, seizures, diarrhea, and urination after the administration of naloxone (1 mg/kg). These findings suggest that the antinociceptive activity of tramadol in mice is due to activation of opioid and nonopioid mechanisms, and as opposed to morphine, is not likely to induce tolerance and physical dependence.

Effect of acute and chronic tramadol on [3H]-5-HT uptake in rat cortical synaptosomes

1. Tramadol hydrochloride is a centrally acting opioid analgesic, the efficacy and potency of which is only five to ten times lower than that of morphine. Opioid, as well as non-opioid mechanisms, may participate in the analgesic activity of tramadol. 2. [3H]-5-hydroxytryptamine (5-HT) uptake in rat isolated cortical synaptosomes was studied in the presence of tramadol, desipramine, fluoxetine, methadone and morphine. Methadone and tramadol inhibited synaptosomal [3H]-5-HT uptake with apparent Kis of 0.27 +/- 0.04 and 0.76 +/- 0.04 microM, respectively. Morphine essentially failed to inhibit [3H]-5-HT uptake (Ki 0.50 +/- 0.30 M). 3. Methadone, morphine and tramadol were active in the hot plate test with ED50s of 3.5, 4.3 and 31 mg kg-1, respectively. At the highest tested dose (80 mg kg-1) tramadol produced only 77 +/- 5.3% of the maximal possible effect. 4. When [3H]-5-HT uptake was examined in synaptosomes prepared from rats 30 min after a single dose of morphine, methadone or tramadol, only tramadol (31 mg kg-1, s.c., equal to the ED50 in the hot plate test) and methadone (35 mg kg-1, s.c., equal to the ED90 in the hot plate test) decreased uptake. 5. Animals were chronically treated for 15 days with increasing doses of tramadol or methadone (5 to 40 mg kg-1 and 15 to 120 mg kg-1, s.c., respectively). Twenty-four hours after the last drug injection, a challenge dose of methadone (35 mg kg-1, s.c.) or tramadol (31 mg kg-1, s.c.) was administered. [3H]-5-HT uptake was not affected in synaptosomes prepared from rats chronically-treated with methadone, whereas chronic tramadol was still able to reduce this parameter by 42%. 6. Rats chronically-treated with methadone showed a significant increase in [3H]-5-HT uptake (190%) 72 h after drug withdrawal. In contrast, [3H]-5-HT uptake in rats chronically-treated with tramadol (110%) did not differ significantly from control animals. 7. These results further support the hypothesis that [3H]-5-HT uptake inhibition may contribute to the antinociceptive effects of tramadol. The lack of tolerance development of [3H]-5-HT uptake, together with the absence of behavioural alterations after chronic tramadol treatment, suggest that tramadol has an advantage over classical opioids in the treatment of pain disorders.

Nonsteroidal anti-inflammatory drugs, traditional opioids, and tramadol: contrasting therapies for the treatment of chronic pain

The treatment of chronic pain is an important function of physicians. In the United States, available drug treatments for chronic pain currently include simple analgesics such as acetaminophen, salicylates and other nonsteroidal anti-inflammatory drugs, traditional opioid drugs, and adjuvant agents (eg, antidepressants, anticonvulsants). Typically, the choice of a drug is made by balancing the indications for treatment, the clinical efficacy of the drug, and its toxicity. An understanding of the mechanism of action of these drugs helps to establish their role in therapy. Tramadol is an effective analgesic that works through a combined mechanism of weak mu receptor binding and the inhibition of serotonin and norepinephrine reuptake. Tramadol has a favorable adverse-effect profile and therefore is likely to have an important role in the management of chronic pain syndromes.

Nalbuphine coadministered with morphine prevents tolerance and dependence

Nalbuphine, an opioid mixed agonist-antagonist, prevents many morphine-related side effects. In this study, we compared the effects of nalbuphine versus naloxone on the prevention of morphine tolerance and dependence in Sprague-Dawley rats. Group 1 received a morphine 5 mg/kg intraperitoneal (I.P.) injection. Groups 2 and 3 received single doses of nalbuphine (0.01 to 5 mg/kg I.P.) or naloxone (1 to 500 microg/kg I.P.) coadministered with morphine (5 mg/kg I.P.), respectively. Group 4 received a saline I.P. injection. Treatments were continued for 4 days. The occurrence of tolerance was estimated by comparing the antinociceptive effect of morphine on Day 1 (Group 1) and Day 5 (each group). The severity of dependence was determined by precipitated withdrawal signs (incidence of diarrhea and teeth chattering) induced by naloxone (10 mg/kg I.P.). We found that coadministration of nalbuphine or naloxone with morphine dose-dependently blocked the development of morphine tolerance and dependence. However, unlike naloxone, nalbuphine did not attenuate the antinociceptive effect of morphine.

A novel approach to the pharmacology of analgesics

To date in the United States when a patient has presented with a complaint of pain requiring some form of pharmacologic relief, the physician has had the choice of two broad classes of drugs: peripherally acting (i.e., NSAID) or centrally acting (i.e., opioid) analgesics. The antidepressant monoamine reuptake inhibitors, particularly when combined with an opioid analgesic, have also proven efficacious in treating certain types of pain conditions. A new approach, available for almost 20 years in Europe and recently approved for use in the United States, is the centrally acting synthetic analgesic tramadol HCI. Preclinical evidence suggests that tramadol produces its antinociceptive effect in animals and analgesic effect in humans through a complementary dual mechanism of action. One mechanism relates to its weak affinity for mu-opioid receptors (6,000-fold less than morphine, 100-fold less than d-propoxyphene, 10-fold less than codeine, and equivalent to dextromethorphan). A metabolite (O-desmethyltramadol; M1) binds to opioid receptors with a greater affinity than the parent compound and could contribute to this component. However, in most animal tests and human clinical trials, the analgesic effect of tramadol is only partially blocked by the opioid antagonist naloxone, suggesting an important nonopioid mechanism. This nonopioid mechanism possibly relates to an increase in central neuronal synaptic levels of two neurotransmitters, 5-hydroxytryptamine (5-HT; serotonin) and norepinephrine. The opioid and nonopioid mechanisms appear to combine in a supra-additive manner in several tests of antinociception, but only in an additive or even counteracting manner in measures of adverse-effect liability. In sum, the apparent dual mechanism of action of tramadol suggests a possible new approach to pain relief.

A risk-benefit assessment of tramadol in the management of pain

Tramadol is a cyclohexanol derivative with mu-agonist activity. It has been used as an analgesic for postoperative or chronic pain since the late 1970s, and became one of the most popular analgesics of its class in Germany. International interest has been renewed during the past few years, when it was discovered that tramadol not only acts on opioid receptors, but also inhibits serotonin (5-hydroxytryptamine; 5-HT) and noradrenaline (norepinephrine) reuptake. This review aims to provide a risk-benefit assessment of tramadol in the management of acute and chronic pain syndromes. Tramadol has been used intraoperatively as part of balanced anaesthesia. Such use is under discussion, however, as it was associated with a high incidence of intraoperative recall and dreaming, and postoperative respiratory depression has been described after intraoperative administration of high doses. Postoperatively, intravenous and intramuscular tramadol has been used with good efficacy. Analgesic doses were comparable with pethidine (meperidine) and 10 times higher than morphine. Nausea and vomiting were the most frequently reported adverse effects. In controlled studies, haemodynamic and respiratory parameters were only minimally impaired. The risk of severe respiratory depression in typical dosages is negligible in comparison with other opioids used for postoperative pain management. Tramadol has been used with good results for the management of labour pain without respiratory depression of the neonate. It was also effective for the treatment of pain from myocardial ischaemia, ureteric colic and acute trauma. Good results have been published for cancer pain management with tramadol in several studies. The potential for abuse or addiction seems to be minimal, and serious complications have not been reported. For patients with severe pain, the efficacy of morphine is superior, and most patients with adequate analgesia from tramadol had to be changed to a more potent opioid after a few weeks due to increased nociceptive input during tumour progression. Tramadol can be recommended as a safe and efficient drug for step II according to the World Health Organization guidelines for cancer pain management.

Contribution of monoaminergic modulation to the analgesic effect of tramadol

1. In humans, the central analgesic effect of tramadol 100 mg orally is only partially reversed by the opioid antagonist naloxone (0.8 mg intravenously). As suggested by in vitro and animal data tramadol analgesia may thus result from an action on opioid as well as monoaminergic pathways. We therefore investigated the effect of alpha 2-adrenoceptor antagonism with yohimbine on tramadol analgesia. 2. Healthy volunteers (n = 10) received tramadol (100 mg orally), followed (+3 h) by yohimbine (0.1 mg kg-1 intravenously), and yohimbine + naloxone (0.8 mg intravenously) and their respective placebo according to a randomized, double-blind crossover, placebo (P) controlled design. Analgesia was assessed over 8 h by subjective pain threshold (pain intensity numerical scale--PINS) and objective pain threshold (RIII nociceptive reflex--RIII) monitoring. 3. Tramadol induced a significant increase in both pain thresholds. Peak analgesic effect was observed at 3.7 h (RIII + 39.6 +/- 3.9% and PINS 50.1 +/- s.e.mean 5%) and the analgesia lasted about 6 h. 4. Yohimbine significantly reversed the analgesic effects of tramadol for 2.8 h with a maximum decrease of 97 +/- 4% (RIII) and 67 +/- 12% (PINS), whereas the addition of naloxone abolished tramadol effects throughout the study period with a decrease of 90 +/- 6% (RIII) and 79 +/- 9% (PINS), P < 0.05). 5. Yohimbine alone did not significantly reduce pain thresholds. 6. alpha 2-Adrenoceptor antagonism reverses tramadol effects, thus pointing to the significant role of monoaminergic modulation and the synergy with opioid agonism in tramadol antinociception after a single oral dose.

Attenuation of c-Fos expression in the rat lumbosacral spinal cord by morphine or tramadol following noxious colorectal distention

We have previously reported that repetitive, noxious colorectal distention (CRD) induces c-Fos in the lumbosacral spinal cord. This study examined the effects of the analgesics morphine and tramadol on c-Fos expression resulting from noxious CRD in the rat. Pre-treatment (30 min or 1 min, i.v.) with morphine (1.25 mg/kg-5.0 mg/kg) or tramadol (1 mg/kg-20 mg/kg) dose-dependently attenuated c-Fos expression to CRD in all areas of the L6-S1 spinal gray matter. The highest dose of morphine was equipotent to the highest dose of tramadol. Repetitive dosing (1/4 of the greatest dose every 30 min) was as effective as a single bolus dose for both drugs. The visceromotor response to CRD was dose-dependently attenuated by tramadol and was reversed by naloxone. However, the dose of tramadol that eliminated the visceromotor response (7% of control) reduced the c-Fos expression to 47% of control. These results demonstrate that these two analgesics attenuate immediate-early gene expression and the visceromotor response to a noxious visceral stimulus and suggest that complete attenuation of c-Fos expression is not necessary for these compounds to produce analgesia to a noxious visceral stimulus.

Quantitative sensory examination of epidural anaesthesia and analgesia in man: effects of pre- and post-traumatic morphine on hyperalgesia

The objectives of the study were: (1) comparison of hypoalgesic effects of pre- and post-traumatic epidural morphine (EM) on primary and secondary hyperalgesia, and (2) comparison of EM hypoalgesia in normal and injured skin. Burn injuries (25 x 50 mm rectangular thermode, 47 degrees C, 7 min) were produced on the calves of healthy volunteers, at 2 different days at least 1 week apart. In randomized order, the subjects received 4 mg of EM administered via the L2-L3 intervertebral space on one day and no treatment on the other day. One calf was injured 30 min prior to and the other calf 2.5 h after administration of morphine. Hence, the calf injured prior to morphine administration was a model of postinjury treatment, and the calf injured after morphine administration, a model of pretraumatic treatment. The timing of injuries was identical on the morphine treatment and control days. The injuries induced decrease in heat pain detection and tolerance thresholds within the area of injury (area of primary hyperalgesia) as well as reduction of areas of allodynia for brush and pinprick surrounding the injury (area of secondary hyperalgesia). Both pre- and post-traumatic administration of EM increased heat pain detection and tolerance thresholds, and decreased by approximately 50% the areas of secondary hyperalgesia 2.5 h postinjury. The effects of morphine were naloxone (NAL)-reversible (0.1 mg/kg, i.v.). There was no significant difference between pre- and post-traumatic administration of morphine on the effect of either primary or secondary hyperalgesia. EM increased the heat pain detection threshold more within the injury than at a corresponding non-injured site. There was no significant difference in the effect of morphine on heat pain tolerance in injured and non-injured skin. Following NAL, the areas of secondary hyperalgesia expanded beyond control size. It is suggested that the major effect of EM on secondary hyperalgesia is inhibition of C fibre-mediated activity which maintains the altered response properties of central neurons responsible for secondary hyperalgesia. Possible mechanisms of action of NAL in enhancement of hyperalgesia are discussed.

The pharmacology of tramadol

(+/-)-Tramadol is a central analgesic with low affinity for opioid receptors. The rate of production of its M1 metabolite (O-demethyl tramadol) is influenced by debrisoquine-type polymorphism, and this metabolite shows a higher affinity for opioid receptors than the parent drug. Experimental and clinical data suggest that tramadol may also exert its analgesic effect through direct modulation of central monaminergic pathways. Indeed, after a single oral dose, the role of the mu-receptor agonist component of the antinociceptive effect of tramadol appears to be minor, with most of the analgesic effect being attributable to nonopioid properties of the parent compound. Approximately 2-fold accumulation of the parent compound and the M1 metabolite may be expected during multiple dose treatment. The duration of analgesic effect after a single oral dose of tramadol 100 mg is about 6 hours. Clinical experience has confirmed that tramadol is an effective and relatively safe analgesic that may be of value in several pain conditions not requiring treatment with strong opioids.

Tramadol. A preliminary review of its pharmacodynamic and pharmacokinetic properties, and therapeutic potential in acute and chronic pain states

Tramadol is a centrally acting analgesic which possesses opioid agonist properties and activates monoaminergic spinal inhibition of pain. It may be administered orally, rectally, intravenously or intramuscularly. In patients with moderate to severe postoperative pain, intravenous or intramuscular tramadol has generally proved to be of equivalent potency to pethidine (meperidine) and one-fifth as potent as nalbuphine. Intravenous tramadol 50 to 150mg was equivalent in analgesic efficacy to morphine 5 to 15mg in patients with moderate pain following surgery; however, when administered epidurally tramadol was one-thirtieth as potent as morphine. Tramadol has demonstrated efficacy in a few studies in the short term treatment of chronic pain of various origins. Orally administered tramadol was found to be an effective analgesic in step 2 of the World Health Organization's guidelines for the treatment of patients with cancer pain. Tramadol is well tolerated in short term use with dizziness, nausea, sedation, dry mouth and sweating being the principal adverse effects. Respiratory depression has been observed in only a few patients after tramadol infusion anaesthesia. When used for pain relief during childbirth, intravenously administered tramadol did not cause respiratory depression in neonates. The tolerance and dependence potential of tramadol during treatment for up to 6 months appears to be low, although the possibility of dependence with long term use cannot be entirely excluded. Thus, evidence to date of the analgesic effectiveness of tramadol combined with a low respiratory depressant effect and low dependence potential in short term use, suggests that the drug may become a useful alternative to the opioid analgesics currently available for the treatment of patients with moderately severe acute or chronic pain.

Increased in vivo release of calcitonin gene-related peptide-like material from the spinal cord in arthritic rats

Possible alterations in spinal systems containing calcitonin gene-related peptide (CGRP) due to polyarthritis were assessed in rats 3-4 weeks after an intradermal injection of Freund's adjuvant in the low back. The tissue levels of CGRP-like material (CGRPLM) were approximately 50% higher in the dorsal zone of the spinal cord and dorsal root ganglia at both the cervical and lumbar (but not thoracic) segments in polyarthritic rats than in age-paired control animals. In addition the rate of the spinal release of CGRPLM determined through an intrathecal perfusion procedure in halothane-anaesthetized animals was approximately 15-fold higher in polyarthritic rats than in controls. The blockade of mu-opioid receptors by intrathecal perfusion with 10 microM naloxone produced a larger increase in the spontaneous CGRPLM outflow in polyarthritic rats than in age-paired controls. Furthermore, the stimulation of mu-opioid receptors by intrathecal perfusion with 10 microM DAGO significantly inhibited the spinal outflow of CGRPLM only in polyarthritic rats. These data indicate that CGRP-containing primary afferent fibres are markedly activated in chronic suffering polyarthritic rats. This activation occurs in spite of an increased tonic inhibitory control by endogenous opioids acting at mu receptors.

The highly selective delta agonist BUBU induces an analgesic effect in normal and arthritic rat and this action is not affected by repeated administration of low doses of morphine

The effect of various doses of the selective delta agonist BUBU (Tyr-D-Ser(O-t-butyl)-Gly-Phe-Leu-Thr(O-t-butyl) on the vocalization threshold to paw pressure were compared in normal and arthritic rats, a suitable clinical model of chronic pain. In both group of rats, the intravenous administration of BUBU (6, 9, 12 mg/kg in normal and 1.5, 3, 6 mg/kg in arthritic rats) led to significant antinociceptive effects. The same dose of BUBU (6 mg/kg i.v.) produced a much more potent antinociceptive effect in arthritic than in normal rats, and a dose as low as 1.5 mg/kg produced a significant analgesic effect in the arthritic animal, whereas at 3 mg/kg BUBU was ineffective in normal rats. The analgesic effects of BUBU (9 mg/kg in normal and 3 mg/kg in arthritic rats) were completely prevented by the selective delta antagonist naltrindole (1 mg/kg i.v. a dose devoid of analgesic potency per se), while they were not affected by the selective mu antagonist naloxone (0.05 mg/kg i.v.). In addition, 3 mg/kg i.v. of BUBU remained effective in morphine tolerant arthritic rats. These results suggest that delta opioid receptor activation can modulate the transmission of cutaneous mechanical nociceptive information in rats, especially in inflammatory pain conditions.

A comparison of epidural tramadol and epidural morphine for postoperative analgesia

The present study compared epidural tramadol with epidural morphine for postoperative analgesia in 20 patients undergoing major abdominal surgery. Intraoperatively, the patients were anaesthetized by a balanced technique of general anaesthesia combined with lumbar epidural lidocaine. In ten of the patients 100 mg tramadol diluted in 10 ml normal saline was also injected epidurally, while 4 mg epidural morphine was used in the other ten patients. In all patients, the visual analogue pain score, PaO2, PaCO2 and respiratory rate were monitored every hour for the first 24 hr postoperatively. In both the tramadol and morphine groups, the mean hourly pain scores ranged from 0.2 +/- 0.6 to 1.4 +/- 2.5 throughout the period of observations. However, the mean PaO2 was decreased postoperatively in the epidural morphine group, while no change was observed in the epidural tramadol group. The maximal decrease of PaO2 in the epidural morphine group was observed at the tenth hour postoperatively, when it decreased to 72.8 +/- 10.3 mmHg. This was not associated with any increase in PaCO2 or a decrease of respiratory rate, suggesting that hypoxaemia rather than hypercarbia or decreased respiratory rate may be an earlier indicator of respiratory rate, suggesting that hypoxaemia rather than hypercarbia or decreased respiratory rate may be an earlier indicator of respiratory depression in patients breathing room air without oxygen supplementation. The absence of clinically relevant respiratory depression following epidural tramadol compared with epidural morphine may be attributed to the different mechanisms of their analgesic action. The results suggest that epidural tramadol can be used to provide prolonged postoperative analgesia without serious side effects.

Effects of the central analgesic tramadol on the uptake and release of noradrenaline and dopamine in vitro

1. The centrally acting analgesic, tramadol, has low affinity for opioid receptors and therefore presumably other mechanisms of analgesic action. Neurotransmitter release and uptake experiments were used to characterize the effects of tramadol on the central noradrenergic and dopaminergic systems. 2. Tramadol inhibited the uptake of [3H]-noradrenaline into purified rat hypothalamic synaptosomes with an IC50 of 2.8 microM; the (-)-enantiomer was about ten times more potent than the (+)-enantiomer. Results with the principal metabolite O-desmethyltramadol were very similar. Inhibition of dopamine uptake into purified rabbit caudate nucleus synaptosomes was very weak with 62% inhibition of 100 microM. 3. Rat occipital cortex slices were preincubated with [3H]-noradrenaline and rabbit caudate nucleus slices with [3H]-dopamine, then superfused and stimulated electrically. Tramadol, 1 and 10 microM, enhanced the stimulation-evoked [3H]-noradrenaline overflow by 25 and 69%, respectively; the (-)-enantiomer was more potent than the racemate or the (+)-enantiomer. Tramadol, 10 microM, had no effect on dopamine release. 4. The effects of tramadol on the stimulation-evoked [3H]-noradrenaline release were abolished when uptake sites were already blocked by a high concentration of cocaine. 5. The metabolite O-desmethyltramadol showed a slight facilitation of the stimulation-evoked noradrenaline release; the effect was more pronounced in the presence of a high concentration of naloxone. In the presence of cocaine, inhibition of the release was observed similar to the effect of morphine but less potent. 6. The results show that tramadol blocks noradrenaline uptake with selectivity as compared to dopamine uptake. The interaction with the noradrenaline transporter is stereoselective. The principal metabolite O-desmethyltramadol shows in addition to noradrenaline uptake inhibition, opioid inhibition of noradrenaline release.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Evidence for a noradrenergic component in the antinociceptive effect of the analgesic agent tramadol in an animal model of clinical pain, the arthritic rat

The analgesic agent tramadol has a potent antinociceptive effect in arthritic rats. In the present study, the actions of the selective alpha 2-adrenoceptor antagonists yohimbine and idazoxan on this antinociceptive effect were tested in arthritic rats, using vocalization thresholds to paw pressure as a nociceptive test. The antagonists were administered 30 min before tramadol, at doses (0.5 and 1 mg/kg i.v.) without action per se, but which prevented the antinociceptive action of the prototypic alpha 2-adrenoceptor agonist clonidine (0.1 mg/kg i.v.) in these animals. The potent antinociceptive effect of tramadol (1 mg/kg i.v.) was significantly decreased (mean total effect reduced about 2-fold) by yohimbine and idazoxan. In alpha 2-adrenoceptor antagonists-pretreated arthritic rats, the effect of tramadol was almost abolished when tramadol was coinjected with the opioid antagonist naloxone. In addition to the involvement of opioid receptors, these results provide evidence for a noradrenergic component to the antinociceptive action of tramadol in this model of clinical pain.